OK, there is a problem here. They are either playing games or idiots.
They state they completed successful testing with UDP and now want to do testing with TCP. There is no chance this would work with UDP other than sheer luck (or an incorrect test set up).
For this purpose, the primary difference between TCP and UDP is how they handle the situation where data is sent but not received.
In both cases, the data to send is broken into smaller pieces called packets. Those packets are given a number (so they can be reassembled) and then sent to the receiver. The reason they are given a number for reassembly is that each packet might follow a different route over the internet and that could result in packets showing up out of order or being lost. That last part is where we get to that primary difference.
In TCP, the packets received are ACK'd (an acknowledgment or receipt response sent back to the sender). If the ACK is not received the packet will be resent. Thus, it is said that TCP is guaranteed delivery.
In UDP, the packets are received without an ACK. Thus, it's very possible that a UDP packet won't be received. For the video game players amongst you, if you are player a first person shooter online and you see another player (or yourself) suddenly transported to an unexpected location then it's very likely UDP packets were lost.
OK, so why does this matter? Without having the entire encrypted message it can't be decrypted. Furthermore, if we assume we are living in a fantasy world and all the UDP packets will always show up then there would be no reason to test TCP.
